The invention relates to a drive device for manually driving a blind, roller shutter or the like equipped with a winding shaft, comprising an epicyclic reduction gearbox between a manual driving means and the winding shaft of the blind.
Such a drive device is known from patent EP 0 372 803, the content of which is incorporated by reference, where it is used for driving a venetian blind by means of a chain driving a chain wheel secured to the sun wheel of a simple epicyclic reduction gearbox comprising a fixed annulus gear and a set of planet pinions carried by a planet carrier constituting the output of the reduction gearbox. The reduction ratio of the reduction gearbox is, for example, 3 to 1.
One of the advantageous properties of an epicyclic or planetary reduction gearbox is the absence of radial resultant of transmission and therefore less stress on the components.
In document EP 0 372 803, the compact nature of the planetary reduction gearbox is brought to the fore. This compact nature is, however, relative. In the device described in the mentioned document, the reduction gearbox is mounted on the outside of the casing housing the winding shaft of the Venetian blind. Arranging the reduction mechanism on the outside of the casing is not only unattractive but also entails the manufacture of two different products, one intended to be operated from the right-hand end and the other intended to be operated from the left-hand end. This is why it is preferable for the reduction mechanism to be able to be housed inside the casing so that there is no longer any lateral appendage and so that the casing can be mounted either way around, with the reduction mechanism at its left-hand end or at its right-handed.
Experience also shows that for this type of use, a reduction ratio less than 3 is preferable because it makes use somewhat more comfortable on account of the movement of the blind that is obtained with respect to the effort and movement required of the user.
If there is a desire to house the reduction gearbox inside the casing, then one is immediately confronted with problems of embodiment. The problems are that, given that it is desirable to obtain a low reduction ratio, the difference in the pitch diameters of the sun wheel and of the annulus needs to be reduced, and the pitch diameter of the planet pinions has therefore to be reduced. Now, if the pitch diameter of the planet pinions is reduced, then the number of their teeth has also to be reduced, which cannot be done indefinitely, or their module has to be reduced, and this leads to them being weaker. To maintain an acceptable number of teeth on the planet pinions, one is therefore forced to increase the number of teeth on the annulus gear and therefore to increase the overall bulk of the mechanism. Thus, for a planetary gear train of the type described and depicted in document EP 0 372 803, the ratio of 3 to 1 mentioned can be obtained by means of a sun wheel with 20 teeth, planet pinions with 10 teeth and an annulus gear with 40 teeth. To change to a ratio of 2.7 while keeping planet pinions with 10 teeth, the sun wheel has to have 29 teeth and the annulus gear has to have 49 teeth; thus, for a module of 1, the outside diameter of the mechanism needs to be increased by 9 mm.
The problem is therefore one of obtaining a low transmission ratio with a small outside diameter, for example a diameter of 20 mm.
Planetary or epicyclic reduction gearboxes are generally used because of the high reduction ratios that can be obtained with such reduction gearboxes. As far as the knowledge of those skilled in the art in the field of gears, and more particularly planetary reduction gearboxes, is concerned, reference can be made to the very comprehensive work by G. Henriot entitled xe2x80x9cEngrenages, conception, fabrication, mise en oeuvre [Gears, design, manufacture and use]xe2x80x9d, 7th edition. In chapter 13.3 on pages 545 to 553, Henriot deals with composite gear trains. A composite gear train is formed of the juxtaposition of two simple planetary gear trains. Henriot shows the 12 combinations that can be obtained with two simple planetary gear trains of type I, but it is also possible to obtain 12 combinations by combining simple planetary gear trains of type II, III or IV or by combining the types with one another. For each combination, there are therefore 10 options for combining the types, namely I-I, I-II, I-III, I-IV, II-II, etc., which represents 120 combinations. Depending on the choice of input, fixed point and output of the reduction gearbox, each combination can give rise to 6 different couplings, which gives a total of 720 options.
Overcoming the existing prejudices regarding the contradiction there is between the set conditions, namely a reduction ratio less than 3 and a small diameter, the inventor asked himself whether there might be one possible combination of simple planetary gear trains that would allow said conditions to be satisfied and have a simple architecture so as to make it easier to manufacture and so as to limit its cost.
After numerous calculations and selection operations, it was found that two reduction gearboxes allow the stated objectives to be achieved. These two reduction gearboxes have common characteristics, namely the fact that the simple planetary gear trains are of type I, that the sun wheels of the two simple planetary gear trains are linked and that one annulus gear is fixed and the other can rotate.
Remember that a simple planetary gear train of type I consists of a sun wheel, a planet carrier and its planet pinions, and an annulus gear.
More specifically, the manual drive device according to the invention is one wherein the epicyclic gearbox consists of the juxtaposition of two simple planetary gear trains of type I the sun wheels of which are mechanically linked so as to be driven simultaneously at the same speed and the annulus gear of the first planetary gear train of which is fixed and the other annulus gear of which can rotate, and which device has a reduction ratio less than or equal to 3.
In one of the embodiments, the planet pinions of the two gear trains are carried by a common planet carrier constituting the input of the reduction gearbox and the annulus gear of the second gear train constitutes the output of the reduction gearbox.
In the other embodiment, the planet carrier of the first planetary gear train is secured to the annulus gear of the second gear train and the planet carrier of the second gear train constitutes the output of the reduction gearbox, the input to the reduction gearbox being on the sun wheels.
For a reduction gearbox outside diameter of 20 mm, it is possible, in both embodiments, to obtain a reduction ratio less than 3, for example of 2.6.